Table 3. Age dependence of SDNN and RMSSD and origin of matched PWI.

For three similarly sized age groups the fractions of matched PWI derived from each of the three accelerometer axes are reported, showing the y axis data is used for more than half of all PWI correctly associated with RRI. The mean values of the HRV parameters SDNN and RMSSD and the mean PTT as derived from matched RRI and PWI are shown for comparison with literature [35, 39]. Regarding SDNN and RMSSD, all differences between the young age group and the other two groups are highly significant (p ≤ 0.002), while no significant differences occur between the intermediate and the elderly group. The results indicate that the reduction in SDNN and RMSSD with age is similar in RRI (as derived from the ECG) and PWI (as reconstructed through wrist actigraphy). The differences between the mean PPT values of the young group and the other two groups are weak but still highly significant (p = 0.004 and p < 0.001, respectively), but also not significant between the intermediate and the elderly group.

age range	18-45 y	46-56 y	57-80 y
number of subjects	117	114	120
fraction for x axis	0.13	0.15	0.09
fraction for y axis	0.51	0.53	0.52
fraction for z axis	0.35	0.32	0.39
SDNN from RRI	78.5 ms	65.9 ms	65.7 ms
SDNN from PWI	83.5 ms	72.6 ms	72.3 ms
RMSSD from RRI	54.5 ms	39.8 ms	39.5 ms
RMSSD from PWI	71.9 ms	61.6 ms	61.7 ms
mean PTT	216.9 ms	206.7 ms	200.7 ms